Vacuum-UV fluorescence spectroscopy of CCl₃F, CCl₃H and CCl₃Br in the range 8–30 eV

Abstract

The fluorescence spectroscopy of CCl₃X (X=F, H, Br) in the range 200–700 nm is reported, using vacuum-UV radiation in the range 8–30 eV from a synchrotron as a tunable photoexcitation source. Excitation spectra, with undispersed detection of the fluorescence, have been recorded at the Daresbury UK source with a resolution of 0.1 nm, corresponding to an average energy resolution of ca. 0.015 eV. Dispersed emission spectra in the range 200–700 nm have been recorded at the BESSY 1 Germany source with an optical resolution of 8 nm, following photoexcitation at the energies of the peaks in the excitation spectra. Action spectra, in which the vacuum-UV energy is scanned with detection of the fluorescence at a specific wavelength, have also been recorded at BESSY 1 with a resolution of 0.3 nm; thresholds for production of a particular excited state of a fragment are then obtained. Using single-bunch mode, lifetimes of all the emitting states that fall in the range ca. 3–100 ns have been measured. For photon energies in the range 8–12 eV, emission is due to both CCl₂ ùB₁–X̃¹A₁ and CXCl ùA″–X̃¹A′. These products form by photodissociation of low-lying Rydberg states of CCl₃X, and the thresholds for their production therefore relate to energies of the Rydberg states of the parent molecule. It is not possible to say whether the other products form as two halogen atoms or a diatomic molecule. For energies in the range 13–17 eV, emission is due to diatomic fragments; CCl A²Δ, CF B²Δ, CH B²Σ^- and A²Δ, CBr A²Δ, and Cl₂ D′ 2³Π_g. From their threshold energies, there is now accumulated evidence that the excited state of CCl or CX forms in association with three isolated atoms. Our results yield no information on whether the three bonds in CCl₃X* break simultaneously or sequentially. In the range 13–17 eV, Cl₂^* almost certainly forms in conjunction with ground-state CX+Cl. This ion-pair state of Cl₂ also forms at higher excitation energies around 20 eV, probably with atomic products C+X+Cl. In no cases is emission observed from excited states of either the CCl₃ radical or the parent molecular ion, CCl₃X⁺.

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